The present invention relates to display devices, such as driving an active matrix electrophoretic display.
Displays, such as liquid crystal (LC) and electrophoretic displays include particles suspended in a medium sandwiched between a drive or pixel terminal and a common terminal. The pixel terminal includes pixel drivers, such as an array of thin film transistors (TFTs) that are controlled to switch on and off to form an image on the display. This conventional method of driving a display is referred to as scan line driving. The voltage difference (VEink=VCE−Vpx as shown in FIGS. 3 and 5) between a TFT or the pixel terminal 101 and the common terminal 102, which is on the viewer's side of the display, causes migration of the suspended particles, thus forming the image. Displays with an array of individually controlled TFTs or pixels are referred to as active-matrix displays.
In order to change image content on an electrophoretic display, such as from E Ink Corporation for example, new image information is written for a certain amount of time, such as 400 ms to 1000 ms. As the refresh rate of the active-matrix is usually higher, this results in addressing the same image content during a number of frames, such as for a frame refresh rate of 50 Hz, 20 to 50 frames. Typically, a frame represents a fixed number of times each row in a display is addressed, typically a single time.
Electrophoretic active matrix displays comprising ordered rows of display pixels are applied in many applications such as e-readers. The individual pixels in the rows are addressed via column voltages transmitted through column electrodes that are selectively transmitted to the pixels via row addressing voltages on row electrodes that may switch the pixel. Although this text refers generally to E Ink displays as examples of electrophoretic displays, it is understood that the invention can be applied to electrophoretic displays in general, such as for example SiPix displays, where the microcups are filled with white particles in a black fluid. Circuitry for driving displays, such as electrophoretic displays, is well known. Such circuitry is described in U.S. Pat. No. 5,617,111 to Saitoh, International Publication No. WO 2005/034075 to Johnson; International Publication No. WO 2005/055187 to Shikina; U.S. Pat. No. 6,906,851 to Yuasa; U.S. Patent Application Publication No. 2005/0179852 to Kawai; U.S. Patent Application Publication No. 2005/0231461 to Raap; U.S. Pat. No. 4,814,760 to Johnston; International Publication No. WO 01/02899 to Albert; Japanese Patent Application Publication Number 2004-094168; and WO 2008/054209 and WO2008/054210 to Markvoort, each of which is incorporated herein by reference in its entirety.
The grey level of a pixel will be referred to as the ‘pixel state’ P, and its value is measured for example by the reflectivity of the pixel. The pixel state P can be generally distributed on an equidistant partition of a dynamic range between the two extreme pixel states of the pixel (e.g., black and white states).
It will be appreciated that the term “equidistant partition of the dynamic range” may relate not only to a physically equal partition, but also to an equidistant partition as perceived by a human eye. It will be appreciated that for this purpose a known human eye sensitivity curve may be used for defining said partition.
It is recognized in the art that reflectance (R) is proportional to power and expressed in Cd/m2. The reflectance can be measured as a function of the wavelength of the light. The average reflectance between a wavelength of 350 nm and 780 nm is defined as the total reflectance of the visible light. The relative reflectance is expressed in % with respect to a reference (white for example). Luminance (Y) is the light sensitivity of human vision in Cd/m2. It is derived from reflectance as a function of the wavelength by a convolution with the eye sensitivity curve. The average value is the total luminance of the visible light. The relative luminance is expressed in % and is the luminance with respect to a reference (white for example).
Lightness (L*) is the perceptual response to the relative luminance in % and has the usual ICE definition:L*=116(R/Ro)1/3−16,wherein R is the reflectance and Ro is a standard reflectance value. A delta L* of unity is taken to be roughly the threshold of visibility. Grey levels in a display are preferably generated equidistant with respect to lightness L*.
The pixel state P of a pixel in an electrophoretic display remains stable when the driving voltage differential VEink is switched off (i.e., VEink=0V). This pixel state stability in the absence of driving voltage is an advantage, as it means that power is only required during a display update. However, the disadvantage is that driving an electrophoretic display is complicated: in order to drive the display one has to know the current pixel states and the intended new pixel states of the display. Typically a so-called Look Up Table (LUT) is used wherein e.g. for 16 grey levels this LUT provides 16×16 waveforms or scan line driving values, giving a recipe for a pixel to be driven from each of the 16 possible grey scales to each of these 16 grey scales.
For an update to have a desired reproducible quality, typically, current e-reader products utilize a 16×16 grey level LUT that for each transition a ‘quality update’ method will execute a reset in a reset phase 910 and a tuning in a tune phase 920. A pixel is driven in one of either the extreme white or the extreme black states for a resetting effect, so that a grey level can be built up in a reliable way from one of the extreme states in the tune phase, minimizing image history. The update time of such a quality update can be approximately 2 to 3 times a switching time of the display effect, which may amount to a relatively long time scale of 500 to 750 ms for E Ink (at 15V and at room temperature).
When the reset phase is skipped a pixel may be directly updated to a desired grey scale. In such a direct update a pixel is updated, with an update path of consecutive states, between the initial state and an update state that is monotonically changing to minimize the display response time; which is quicker than the above quality update but may involve a certain inaccuracy—especially when the updating is repeated over longer times. The update time of a direct update can be about a single switching time of the display effect, which may amount to a somewhat shorter update time of about 250 ms for E Ink (at 15 V and at room temperature).
Because of the underlying physics of the electrophoretic display material, the display typically requires relatively long times to adjust to a desired new grey level or pixel state. However, for certain applications, especially faster display changes are desired (e.g., in the response of the display to an input device, such as keyboard or cursor). Thus, a faster drive method is sought, where it is still possible to update the electrophoretic display to any desired pixel state, but yet seeking the potential advantage of a quicker response.